The present invention relates, in general, to the art of induction charging of liquid spray particles in electrostatic spray coating apparatus, and more particularly to a self-adjusting power supply for such apparatus.
The principle of providing an electrostatic field for charging atomized liquid particles and utilizing the charged particles for coating a workpiece has been in general use for many years. A great variety of spray gun configurations have been devised for such systems, some of which work reasonably well and some of which are less than satisfactory. Virtually all of these prior electrostatic devices have in common a spray gun to which is mounted a high voltage electrode disposed adjacent the spray discharge point and carrying an electrical potential in the neighborhood of from 50 to 85 kilovolts; in certain instances, the potential may be as high as 150 kilovolts. The potential on this electrode and the resulting electric field creates a corona discharge condition, producing a region rich in ions through which the spray particles pass. Some of these ions become attached to the spray droplets, producing net electric charges on the particles which may then be directed by the electric field extending between the electrode and an electrically grounded workpiece which, therefore, attracts the particles.
The use of very high potentials on such prior devices has been required to insure that the potential gradient between the spray gun and the target to which the spray particles are directed is sufficient to maintain the conditions under which ionization, and thus charging of the particles, can be created and maintained. The high voltages utilized in these systems produce hazardous working conditions, however, for many of the spray coating materials employed in such systems are flammable, and in some cases highly so, producing the likelihood of a fire or explosion if an arc or spark of sufficient intensity should be produced by moving the spray gun and its corona discharge electrode too close to a grounded object. To insure that no sparking or other discharges occur in such systems it has been found necessary to impose strict limits on the spacing of charging electrodes from the workpiece to be coated, resulting in a decrease in the efficiency of the operation since the average potential gradient between the electrode and the target is correspondingly limited.
In cases where the material being sprayed is not of a nature to create the danger of fire or explosion, problems still exist since the high voltage used in such systems can result in severe electrical shocks to operating personnel. Even though the current flow is below a level which might cause permanent injury or death, nevertheless the shocks produced by such devices are unpleasant, and often can have serious consequences. The high voltages may also create an excessive current flow in the form of ions which may travel to nearby objects, resulting in an undesired charge build up on objects that are not adequately grounded, including the spray gun itself on occasion. Such a build up on the spray gun can result in clogging and premature shut down for cleaning.
The creation of ionized particles in a corona device, and the subsequent motion of the particles from the spray gun to a target such as a grounded workpiece, produces a substantial current flow and this current, together with the very high potentials required for the production of corona effects, imposes a requirement for a relatively large capacity power supply. Conventional high voltage power supplies having sufficient capacity for such applications are not only bulky and heavy, but are quite expensive. Further, such power supplies normally are separate units connected to the spray gun by way of a high voltage cable which must be heavily insulated to withstand the high output voltage. Such cables are relatively inflexible and thus are unwieldy and difficult to use, and the combination of a bulky, heavy power supply with heavy, inflexible interconnecting cables substantially restricts the usefulness of the spray gun because of the difficulty of handling and moving it.
In each of U.S. application Ser. No. 634,386 of James E. Sickles, filed Nov. 24, 1975, and U.S. application Ser. No. 548,958, also of James E. Sickles, filed Feb. 11, 1975, there is disclosed a spray coating apparatus which, through the induction charging of spray particles, avoids the problems inherent in the corona discharge apparatus discussed above. Induction charging is accomplished in the devices disclosed in these applications by surrounding the liquid discharged from a spray nozzle with a static electric field which has an average potential gradient in the range of about 10 to 20 kilovolts per inch. In the preferred form, one or more electrodes are spaced about the liquid discharge nozzle, a suitable potential is applied to the electrodes, and the nozzle is held at ground potential. The impedance of the space between the nozzle and the electrodes to which the potential is applied is sufficient to prevent an electrical discharge either in the form of an arc or a corona. The static electric field induces on liquid particles produced within the field an electrical charge having a polarity which is opposite to that of the applied voltage, with the particles carrying quantities of the charge. Because the charges are induced in the particles by a static field at reduced electrode potential without a corona discharge, almost no current flows in such devices, thus allowing a substantial reduction in the current and voltage requirements for the power supply. The resulting charged particles may then be directed at, for example, an electrically grounded workpiece, the particles striking the workpiece to provide a coating of the liquid thereon.
Such induction charging techniques have been found to be particularly useful in spray systems utilizing electrically conductive liquids such as water base paints, since the liquid can be electrically grounded. This eliminates one of the hazards of corona discharge devices which utilize a high voltage source in contact with the liquid being sprayed. In most corona devices, at least a portion of the liquid is at approximately the same high voltage as the electrode, requiring in the case where a conductive liquid is used, that the liquid itself and its supply source be electrically isolated to prevent excessive current flow to ground and consequent shorting of the power supply, and to insure the safety of the operator. The lower voltages and current levels at the electrode, together with the grounding of the liquid supply, in an induction type of system eliminates the problems inherent in high voltage isolated systems, thus reducing the danger of operator injury, providing increased convenience and flexibility in use, reducing the danger of arcing if the spray gun is moved too close to a grounded object, and reducing the intensity of any such arc and the danger of fire if such arcing should occur.
The aforementioned patent application disclose both apparatus and methods for producing inductively charged spray particles utilizing a DC voltage of between about 7 and 25 kilovolts. The devices disclosed therein have been operated successfully to provide greatly improved electrostatic spray coating operations, and from this standpoint have been more than satisfactory. Further, the power requirements imposed by the lower voltage and current ratings of an induction spray operation have led to the development of smaller and less expensive systems and recently to the development of portable power supplies which may be attached to the spray gun itself. This has resulted in a spray gun apparatus which is easier to handle, which is less expensive, which is easily portable, and which has improved safety characteristics. Such power supplies are described in greater detail in copending applications Ser. No. 739,171 filed Nov. 5, 1976, of Charles D. Hendricks, entitled "Detachable Cylindrical Power Supply for Induction Type Electrostatic Spray Gun", and Ser. No. 739,170, filed Nov. 5, 1976, of James E. Sickles, entitled "Detachable Power Supply for Induction Type Electrostatic Spray Gun", both assigned to the assignee of the present application.
Although the relatively low voltages used in induction charging spray guns reduced or eliminated many of the problems that had been inherent in corona discharge spray guns, some of the problems remained. For example, the potential carried by the charging electrodes, although greatly reduced, was still sufficient to produce a substantial shock if the operator should accidently come in contact with it, and the danger of arcing or sparking still existed if the electrode should be moved too close to a target or other grounded object, if a spray build up should occur within the gun that would reduce the distance between the charging electrode and the grounded liquid supply nozzles, or if the conductivity of the liquid being sprayed should change. Attempts were made to control the problem of shock and arcing through the use of a series resistance connected between the power supply and the electrode in the manner of the series resistances used in corona discharge devices. As described in patents such as U.S. Pat. No. 3,048,498 to Juvinall et al., U.S. Pat. No. 2,926,106 to Gauthier, and U.S. Pat. No. 3,169,498 to Juvinall et al., such series resistors typically have been used to limit the current flow from the high voltage source in the event of a short circuit condition at the corona discharge electrode, thereby preventing the dangerous current levels which could be injurious to personnel or which could produce intense sparking that could lead to explosions and fire. Such a limiting resistor is also disclosed in U.S. Pat. No. 3,698,635 of James E. Sickles, this patent teaching its use in an induction charging device for the same purpose.
Such current limiting resistors, as taught in the prior art, are selected at an ohmic value that is sufficiently high to provide the required safety features, but which is sufficiently low to insure that the corona discharge effect is not disturbed, and that a sufficiently high potential gradient is maintained between the spray gun apparatus and the target to insure proper transfer of the spray particles. Thus, as set forth in U.S. Pat. No. 3,048,498 to Juvinall et al., the value of such a resistance should be on the order of 10 megohms per kilovolt, or slightly more, with a value of more than about 100 megohms per kilovolt being undesirable as adversely affecting the operation of the device. As stated in that patent, this relationship obtains throughout all voltage ranges used for electrostatic spray coating. Other patents relating to corona effect devices teach the provision of similar values of resistance, or values of the same order of magnitude, to accomplish the same current limiting purposes, and these teachings were followed in U.S. Pat. No. 3,698,635 to Sickles, which uses a current limiting resistor having an ohmic value of about 125 to 140 megohms with an electrode voltage of about 10 kv.
In spite of these numerous teachings in the prior art concerning the use of a series resistor for current limiting purposes, however, some difficulties have still been encountered in the operation of induction charging spray guns. The principal problems have occurred in maintaining the optimum potential gradient between the charging electrode and the liquid spray, which typically is at ground potential, which is required for maximum droplet charging and deposition of spray particles. Although it is possible to establish the desired potential for the spray gun at the start of a spray operation, it has been found that the collection of even a few liquid particles on the various parts of the gun, including the barrel and supports for the electrode as well as on the nozzle and the electrode itself, can result in a reduction of the effective spacial impedance between the electrode and the liquid spray. This reduction in the spacial impedance increases the voltage gradient across this space, producing an increased current flow, and varying the charge induced on the spray particles. A continued collection of particles can lead to a large voltage gradient, producing an excessive current flow which can result in corona or arcing within the charging zone of the spray gun or between the gun and a target, thereby significantly reducing droplet charge efficiency. When such arcing occurs, the current limiting resistor of the prior art may function to limit the arc current, but the changes in the potential gradient prior to actual arcing are not sufficiently controlled by the current limiting resistor of the prior art to prevent adverse effects on the deposition efficiency of the coating, nor do such resistors prevent undue drains on the power supply. If highly conductive liquids are being sprayed, the foregoing problems are aggravated, and the build-up of material on the gun can cause substantial leakage of current and reduction in the effectiveness of the spray apparatus.
Another problem encountered in the use of prior electrostatic spray guns is the fact that any steps taken to prevent the current leakage described above in the operation of electrostatic spray guns result in a device which has a high capacity for the storage of electrostatic charges even after it has been turned off and the spray operation has stopped, with the result that such spray guns can retain a very high potential. This capacitive effect can produce unpleasant shocks and can create sparks which can do considerable damage if great care is not taken in the handling of the devices. The provision of current limiting resistors in the prior art devices, as discussed above, tends to increase the ability of the spray gun to retain such charges, and in this regard such resistors have been a hinderance rather than a help.
Because of the difficulties which significant variations in the potential gradient from the nominal value can produce in prior systems, it has been found necessary to provide a potentiometer or similar control means on the power supply that permits manual adjustment of the voltage level to accommodate the changing conditions during the operation of the gun. Such a control means requires continual adjustment by the operator and requires acquisition of a considerable degree of skill if a uniform coating is to be obtained. If the operator is not alert to changing conditions, such as variations in the humidity in the air, changes in the conductivity of the liquid being sprayed, the build-up of particles in the spray gun and the like, changes in the potential gradient within the gun or between the gun and the target will lower the deposition efficiency of the coating. When the change is discovered by the operator, it is then necessary to stop the spraying operation and adjust the power supply for a return to optimum conditions, before continuing. Such an arrangement has been found to be not only inconvenient, but unsatisfactory in practice.